This invention relates to the removal of metal contamination from water used to quench articles that have been dipped in molten metal baths. More particularly, this invention relates to the removal of zinc and lead contamination from quench water streams produced in the galvanizing of articles such as chain link fencing and in the annealing and galvanizing of articles such as wire strand.
A number of processes for the treatment of metal articles such as steel chain link fencing, iron or steel wire or copper wire, involve passing the articles through a bath of one or more molten metals, for example, lead, zinc, tin or antimony or mixtures of such metals, and quenching the dipped articles in relatively cool water. The articles are dipped in the molten metal to provide a metal coating thereon. U.S. Pat. No. 2,456,235 describes one such process, in which properly prepared iron or steel is dip-coated in a molten mixture of lead and antimony and then quenched in water. Another example is the tinning of copper wire or other articles, which involves coating the copper with a mixture of lead and tin.
Articles are also dipped in molten metal to anneal them. For example, steel wire is passed through a bath of molten lead maintained at an annealing temperature. The lead does not form a continuous coating on the wire because the wire has not been suitably prepared and fluxed to be wetted by the molten lead. Nevertheless, some molten lead clings to the wire when it is removed from the bath. The wire is then quenched in water, and lead contaminates the water.
The galvanizing of iron and steel by dip-coating in molten zinc is a process of major commercial importance. Galvanized articles such as roofing nails, buckets, fence posts, bailing wire, chain link fence, nuts, bolts, chicken wire, wire mesh, reinforcing plates, etc. are in common use. Such articles are galvanized either on a batch or a continuous basis.
In a typical galvanizing process, an article such as chain link is passed continuously through an acid cleaning bath, a water rinse bath to remove acid, a flux bath, a molten zinc bath, and a quench water bath. To conserve water, it has been customary to circulate water from a source such as a city water line through the zinc quench water tank, then through the water rinse taken where acid is removed from the article, and then to a waste line. Where a lead annealing bath and lead quench water tank are used, a portion of the zinc quench water is directed to the lead quench tank, and then to the waste line. When the quench water mixes with the acidic rinse water in the rinse tank or in the waste line, zinc and lead in the water dissolve.
Efforts to remove such dissolved metal contamination from the effluent have centered on chemcial or electrolytic processes for removing metal from the combined effluent stream.
To this inventor's knowledge, it has not heretofore been appreciated that the metal contamination in the effluent is first present as suspended, particulate zinc or lead in the quench water, which dissolves when the metal-laden quench water is mixed with water from the rinse bath.